A review of X-ray laser development at Rutherford Appleton Laboratory

Recent experiments undertaken at the Rutherford Appleton Laboratory to produce X-ray lasing over the 5-30 nm wavelength range are reviewed. The efficiency of lasing is optimized when the main pumping pulse interacts with a preformed plasma. Experiments using double 75-ps pulses and picosecond pulses superimposed on 300-ps background pulses are described. The use of travelling wave pumping with the approximately picosecond pulse experiments is necessary as the gain duration becomes comparable to the time for the X-ray laser pulse to propagate along the target length. Results from a model taking account of laser saturation and deviations from the speed of light c of the travelling wave and X-ray laser group velocity are presented. We show that X-ray laser pulses as short as 2-3 ps can be produced with optical pumping pulses of approximate to1-ps

A model for opacity measurements across expanding X-ray laser media

A theoretical model has been developed for opacity calculations across an expanding laser produced plasma used as an x-ray laser medium. The assumptions of planar geometry, exponentially decreasing emissivity and absorption coefficient with distance with a step rise at a particular point and a linear with distance plasma bulk velocity profile have been made. The hydrodynamic coupled to the atomic 'EHYBRID' code has been modified to take into account the radiation trapping across the plasma using the net radiative bracket predicted by the above model. The result of the theoretical model as well as the output of the modified EHYBRID code have been employed to solve the radiation transfer equation and to simulate 123 Ne-like and 399 F-like Germanium resonance lines both in time resolved and time integrated modes. We obtain agreement between the simulated spectra and experimental spectra measured during x-ray laser experiments

Short pulse pumped X-ray lasers

Double laser pulses of duration [MATH] and short laser pulses [MATH] superimposed on longer duration background pulses have been shown to efficiently pump lasing in Ne-like and Ni-like ions. For the 75 ps pumping, X-ray laser output without travelling wave pumping is shown to be well-described by a model of ASE output. With 1 ps pumping, the X-ray laser output with different velocity travelling wave pumping is well-fitted with an extension to the ASE model allowing for travelling wave excitation of the gain along the plasma line. The model is used to investigate the production of short ([MATH]) x-ray laser pulses and the effects of non-ideal travelling wave velocities on the X-ray laser output. Resonance line spectra of emission perpendicular to the plasma line are measured and simulated. It is shown that an accurate opacity model for the more intense Ne-like ions is needed to correctly simulate the spectra

Short pulse pumped X-ray lasers

Double laser pulses of duration [MATH] and short laser pulses [MATH] superimposed on longer duration background pulses have been shown to efficiently pump lasing in Ne-like and Ni-like ions. For the 75 ps pumping, X-ray laser output without travelling wave pumping is shown to be well-described by a model of ASE output. With 1 ps pumping, the X-ray laser output with different velocity travelling wave pumping is well-fitted with an extension to the ASE model allowing for travelling wave excitation of the gain along the plasma line. The model is used to investigate the production of short ([MATH]) x-ray laser pulses and the effects of non-ideal travelling wave velocities on the X-ray laser output. Resonance line spectra of emission perpendicular to the plasma line are measured and simulated. It is shown that an accurate opacity model for the more intense Ne-like ions is needed to correctly simulate the spectra

Collisionless shock and supernova remnant simulations on VULCAN

The VULCAN [C. N. Danson et al., Opt. Commun. 103, 392 (1993)] laser at the UK Central Laser Facility is being used for laboratory-based simulations of collisionless shocks. By ensuring that key dimensionless parameters in the experiments have values similar to those of supernova remnants (SNRs), the hydrodynamics and magnetic field of the experiment are scaled to those of a SNR. This makes it possible to investigate experimentally the physics of collisionless magnetized shocks in such objects. The experiments are providing data against which to test current theory. Collisionless shock formation and the interaction of two counterpropagating colliding plasmas permeated by a strong magnetic field are discussed

Saturated x-ray lasers at 196 and 73 Å pumped by a picosecond traveling-wave excitation

Traveling-wave irradiation with a laser pulse of duration ∼1 picosecond has been shown to achieve saturated operation of Ne- and Ni-like x-ray lasers. Gain at 196 Å was confirmed by observation of both forward and backward x-ray laser beams from a germanium plasma under ideal and nonideal traveling-wave conditions at a small signal gain of >40 cm-1. Saturation was observed for targets >4 mm long consistent with a model of laser amplification along the plasma medium and with the output of a detailed ray-tracing post-processor coupled to a hydrodynamic and atomic physics code. Ni-like samarium targets, pumped under ideal traveling-wave conditions exhibited a small signal gain of ∼19 cm-1 at 73 Å with saturation observed for targets 8 mm long

Characterization of x-ray lasers at short wavelengths

Experimental measurements of the optimum pulse delay and pulse irradiance ratio for lasing at 7.3 nm in Ni-like samarium are shown to be explained by a simple model. Experimental x-ray laser outputs are also shown to be well fitted with a simple one-dimensional ASE variation of the laser output with increasing target length through saturation. The ASE model is modified to take account of the two lasing lines present in Ni-like lasing and compared to experimental laser output measurements for Ni-like Dy lasing at 5.86 nm and 6.37 nm. The good agreement of experimental data with the ASE model indicates that refraction, non-uniformity of the laser medium and other problems traditionally associated with x-ray lasing are not significant with double pulse pumping

Measurement of the duration of X-ray lasing pumped by an optical laser pulse of picosecond duration

Measurements of the duration of X-ray lasing pumped with picosecond pulses from the VULCAN optical laser are obtained using a streak camera with 700 fs temporal resolution. Combined with a temporal smearing due to the spectrometer employed, we have measured X-ray laser pulse durations for Ni-like silver at 13.9 nm with a total time resolution of 1.1 ps. For Ni-like silver, the X-ray laser output has a steep rise followed by an approximately exponential temporal decay with measured full-width at half-maximum (FWHM) of 3.7 (±0.5) ps. For Ne-like nickel lasing at 23.1 nm, the measured duration of lasing is ≈10.7 (±1) ps (FWHM). An estimate of the duration of the X-ray laser gain has been obtained by temporally resolving spectrally integrated continuum and resonance line emission. For Ni-like silver, this time of emission is ≈22 (±2) ps (FWHM), while for Ne-like nickel we measure ≈35 (±2) ps (FWHM). Assuming that these times of emission correspond to the gain duration, we show that a simple model consistently relates the gain durations to the measured durations of X-ray lasing